An analog synchronous PSK/FSK demodulator according to the related art includes a mixer to execute frequency conversion on an RF signal received by an antenna, a channel selection filter, and a voltage controlled oscillator (VCO) to supply a local oscillation signal to the mixer and adopts a phase locked loop to supply a control voltage of the VCO from an output of the channel selection filter and lock phases of a VCO frequency and an RF signal frequency.
In this type of demodulator, when there is an interfering wave having large power, the VCO is pulled in an interfering wave frequency, instead of the RF signal frequency. Therefore, interfering wave resistance is not sufficient. For example, even though it is considered that the interfering wave is suppressed by a BPF disposed at a previous stage of the mixer, when the interfering wave frequency approaches an RF signal, an extraordinarily sharp cutoff characteristic that cannot be realized in an external component is required. Therefore, a problem is not resolved.